The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Open enclosure specifications that target the communications equipment market, for example the PICMG (PCI Industrial Computer Manufacturers Group) Advanced TCA (Telecom Computing Architecture) specification, describe building practices that utilize connectors extending beyond the front board module (known as a “blade”) area. The connectors provide the necessary power and signal interconnects from the blade to the system midplane.
These types of connectors typically create gaps between the system midplane and the edge of the front board that engages the connectors. The gaps create unwanted lateral air flow paths between the card slots for the front boards. This is illustrated in FIG. 1, where a rear edge 12 of a front board 10a has connectors 13 that are electrically coupled to connectors 14 extending from a midplane 16. A portion of the front board 10a may also couple to electrical connector 15. The front board 10a and midplane 16 are positioned within an enclosure 17 that typically houses a plurality of front boards in side-by-side fashion, with each front board occupying a slot within the enclosure.
In FIG. 1 the side view of front board 10a (which may be termed blade 1) can be seen to have heat sinks 18 that form a high air flow impedance along the top edge 20a of the front board 10a. Between the rear edge 12 of each front board 10a and the front surface or side 22 of the midplane board 16 (hereinafter simply “midplane 16”) are formed a plurality of gaps 24. Typically several gaps 24 will exist in the plane between the rear edge 12 of the front board 10a and the connectors 14 and 15. The gaps 24 enable cooling air flows 26 being directed through each front board card slot to be diverted through the gaps 24, thus significantly reducing the intended volume of air flow over the front board 10a. FIG. 2 illustrates the air flows 26 along an adjacent front board 10b (i.e., which can be termed blade n+1) caused by heat sinks 28 at a lower edge of the front board 10b. FIG. 3 illustrates a front view looking at successively positioned front boards 10b, 10a and 10c and how the air flows 26 tend to take the paths of least impedance as the air flows come up from the below the center front board 10a and past heat sinks 28 and 29.
Addressing this undesirable lateral air flow through the gaps 24 can be challenging. The air flow path through the enclosure cardage configuration can be very complex, and typically is proportional to the impedance distributions of the available parallel paths. By the term “impedance” it is meant the resistance to air flow. Airflow follows the least restrictive path first or, put differently, the path with the least air flow impedance. A significantly higher portion of the air flow will flow through the lowest impedance path than would be suggested by simple areal proportions of the front board geometry. In essence, a large quantity of air can be diverted through the small gaps 24 if the overall impedances of the other available air flow paths are higher than the flow path impedance through the small gaps 24. If the adjacent front board card slots are not completely isolated from each other, then lateral flow paths will be formed through the gaps 24 and between the slots around the front board boundaries. These lateral paths can lead to significant air flow distribution issues, thus complicating the system integration process and requiring configuration specific thermal testing. As will be appreciated, such configuration specific thermal testing would be time consuming and expensive. Even if such testing was successfully performed, the changing out of one or more front boards at a later date could significantly alter the internal air flow distribution paths within the enclosure, thus necessitating re-testing. Still further, it may be determined that to overcome the negative consequences of the lateral air flows through the gaps 24, that an increased cooling capacity will be needed. The need for increased cooling capacity will necessitate the use of a higher capacity air mover device, or a greater number of air mover devices within the system. This will add to the overall cost of the system and increase the operational cost over time due to increased energy consumption. The use of additional air mover devices will also increase the acoustic noise emissions from the enclosure.
An additional path where the cooling air flow may escape through is at one side of the enclosure. Typically the shelf internal dimensions are not an exact multiple of the slot pitch, which leaves a small internal gap present at one side of the enclosure, as shown in FIG. 1A. For example, the slot pitch in an AdvancedTCA shelf is 30.48 mm wide. The gap in the typical shelf implementations ranges from 5 to 10 mm, representing the 16.4% to 32.8% of the total cross section area of the slot flow path. Because this area is typically almost completely empty of any solid structures, it provides a low impedance parallel air flow path for air to bypass the front board components. Since many front boards have high component density and can contain large cross-section area components, such as hard disk drives, the air flow path through the front board area often represents very high impedance. The air flow takes the lowest impedance path, which can cause substantial amounts of the cooling air flow going through the gap rather than flowing past the entire surface area of the front board. Furthermore, the volume of flow through the gap can be a significantly higher percentage of the total air flow than the relatively small cross sectional gap area would suggest. Obviously, this diverted air flow through the gap does not help in achieving the needed cooling of the components on the front board positioned adjacent to the gap.
Due to the air bypass at the side gap, the shelf air mover devices typically need to be operated at a higher speed to ensure sufficient cooling air flow through the slot cross section where the components reside (i.e., taking into account the lost air flow through the side gap). This can lead to increased acoustic noise as well as increased energy consumption.